The sulfur oxides produced from combustion of sulfur in vehicle fuel inhibit the activity of the noble metal catalyst in the converter for exhaust gas and poison the catalyst irreversibly. Consequently, the exhaust gas comprises unburned non-methane hydrocarbons, nitrogen oxide and carbon monoxide, all of which easily form photochemical smog when catalyzed by sunlight, resulting in acidic rain. Meanwhile, sulfur oxides in the air are one of the major reasons for forming acidic rain. With increasing recognition of environmental protection and stricter environmental regulations, it is believed that decreasing the sulfur content in gasoline or diesel fuel is one of the most important measures to improve the air quality.
In China, most sulfur contained in gasoline comes from heat processed gasoline, which is mainly catalytic cracking gasoline. Therefore, decreasing the sulfur content in the cracking gasoline would facilitate reducing sulfur content of these gasolines. The current standard for gasoline product is GB 17930-2006 “Motor Vehicle Gasoline,” which further restricts the sulfur content of gasoline and requires that by Dec. 31, 2009 the sulfur content of gasoline should be lowered to 50 ppm. Under this circumstance, catalytic cracking gasoline must be desulfurized deeply to meet the environmental requirement.
While lowering the sulfur content of motor vehicle fuel, it should avoid changes in olefin content which leads to a reduction of octane number (including Research Octane Number, ROM and Motor Octane Number, MON) to retain the combustion characteristics of the motor vehicle fuel. Generally, the negative effect on the olefin content is caused by the hydrogenation reaction induced upon removal of thiophene compounds (including thiophenes, benzothiophenes, alkylthiophenes, alkylbenzothiophenes and alkyldibenzothiophenes). Further, it should avoid loss of aromatic hydrocarbons in the cracking gasoline due to saturation under hydrogenation condition. Therefore, the most desirable approach is to desulfurize the gasoline with retaining its octane number.
On the other hand, both hydrodesulfurization and hydrogenation of unsaturated hydrocarbons consume hydrogen, which increases the cost of the desulfurization. Accordingly, there is a need for a method of desulfurization without consuming large volume of hydrogen, for example the S Zorb adsorption desulfurization technology, thereby providing more economical methods for treatment of cracking gasoline or diesel fuel.
Traditionally, a fixed-bed process is usually used for desulfurization in a liquid phase. However, this process is inferior in the homogeneity of reaction and the regeneration of the material. As compared with the fixed-bed process, fluidized-bed process is more advantageous with wide applications prospects in the future because of its better performance in heat transfer and pressure drop. Fluidized-bed reactor is usually provided with granular reactants. However, for most reactions, the reactants do not have enough abrasion-resistance. Accordingly, it is of great significance to find an adsorbent with both excellent abrasion-resistance and desulfurization performance.
Chinese Patent CN 1151333A provides a new adsorbing composition comprising zinc oxide, silica, colloidal oxide and accelerant and a process for making the adsorbent. In the process, fluidizable particles are produced by pressure forming technique, and the particle pore volumes are increased by adding to the colloid a pore forming agent which becomes flammable when heated. The particles prepared by this process are comparatively big, and the particle size is within the range of about 100 to about 300 micron, which is not most favorable for the fluidization process. In addition, particles formed by adding a flammable pore forming agent to increase pore volume are frangible and have low strength.
U.S. Pat. No. 6,150,300, Chinese Patents CN 1130253C and CN 1258396C disclose a granular adsorbent composition comprising mixture of zinc oxide, silica, alumina, nickel or cobalt in a reduced state. The adsorbent is made by first mixing silica, alumina and zinc oxide under shearing, preparing solid particle through granulating machine, and impregnating it with nickel after drying and calcinating the particle. These patents do not disclose the physical-chemical properties, particularly the abrasion-resistance of the adsorbent, although the adsorbent disclosed in these patents has good desulfurization performance.
Chinese Patent CN 1208124C describes a process for making an adsorbent for removal of the sulfide contained in cracking gasoline. The process comprises the steps of impregnating an adsorbent carrier comprising zinc oxide, expanded perlite and alumina with metal accelerants such as cobalt and nickel, and subsequently reducing the accelerant at appropriate temperature. The abrasion-resistance of the adsorbent can be improved by adjusting the level of zinc oxide and binder (primarily alumina) in the adsorbent. Chinese Patent CN 1627988A further discusses in detail the major compounds produced under the reaction conditions. The physical-chemical properties of particles prepared by spray drying method in this patent are more suitable for fluidized-bed. Chinese Patents CN 1856359A and CN 1871063A disclose adsorbents having similar composition and process of making the same.
In the preparation methods of these adsorbents, alumina is added to conglutinate zinc oxide in order to improve the strength of the adsorbent, and expanded perlite is added to ensure the desired physical properties of the adsorbent. However, due to the existence of chemical reactions of zinc oxide during the adsorption desulfurization process, the structure of the adsorbent tends to be collapsed, resulting in lower adsorbent activity. Therefore, it is desirable for the person in the art to have an adsorbent with high desulfurization activity and excellent abrasion-resistance.